The present invention relates to a method of transferring material, preferably particles, from a bulk medium, preferably a fluid and, more particularly, to a method which can be used for printing.
Various filtration and separation techniques are known utilising electrophoretic and similar processes.
GB-A-1 186 184 discloses separation apparatus which combines electrophoresis and bulk flow to effect a separation of one or more components which are uniformly or non-uniformly dissolved or dispersed in one or more relatively continuous fluids to be partly or wholly separated from each other and/or from the fluid or fluids. The fluid flows in an annulus between an inner fixed cylinder and outer rotating cylinder in the presence of an electric field.
U.S. Pat. No. 3,909,383 shows a cleaning process which uses known electrophoretic technology to clean contamination from liquids which are used for developing images in electrophotographic printing systems which use liquid toners. The cleaned liquid can be re-used while the filtered solid is removed for subsequent disposal.
EP-A-0 320 709 discloses an electrophoretic separation technique similar to those mentioned above.
WO-A-88 09210 discloses that, instead of using electrophoresis as a means of cleaning a liquid containing particles of various types, ultrasound can be deployed to drive particles across a flowing liquid such that they can be collected.
GB-A-2 098 498 relates to a separation technique which also uses ultrasound to deflect particles in a flowing liquid in order to clean the liquid. In the method described, a means is employed which causes a drift in the ultrasound field which causes particles to be swept across the flow to a collection point. The contaminant particles can be removed periodically.
In U.S. Pat. No. 4,011,157 the object of this invention is to remove, or filter, particulate matter from a contaminated liquid so that the cleaned liquid can be reused. The method utilises known ultrasonic nebuliser technology to separate liquid ink from particulate contaminant. The cleaned liquid ink (free from particulates) is transferred back to the print station while the solid contamination is collected in a tank for disposal.
WO-A-93 20927 discloses a method which makes use of the known phenomenon of dielectrophoresis to separate particles which are suspended in a liquid which flows through a cell. It is also recognised that this type of system can be used to concentrate particles, although no application of the phenomenon is given.
EP-A-0 307 940 discloses using a laser to trap a biological particle.
FR-A-2 232 192 discloses using an alternating magnetic or electric field to maintain an ink containing magnetic particles in a useable form i.e. prevent it from solidifying, and then using a superimposed static field to eject fluid from the container. The patent specification also teaches that, by making the electrodes or pole pieces pointed, the fluid which is ejected can be concentrated.
CH-A-562 631 shows a mixing and separation technique for particles in a gas and weighing less than 10.sup.-22 g in weight, using donor and acceptor streams and an electric field.
Field Flow Fractionation--JC Goodings Chemical & Engineering News, Vol 66, No. 41, Oct. 10, 1988 summarises many of the techniques listed in the above patent specifications. A fluid containing particles flows through an external field, or gradient such as an electric or magnetic field, or a thermal gradient, which is orthogonal to the flow direction. Different fractions are deposited at different locations in the flow stream.
All of the above are based on known effects and they have been used to separate dispersed particles from a liquid, for example to clean the liquid.
U.S. Pat. No. 4,717,926 & U.S. Pat. No. 4,928,125 describe printing techniques using electric fields and magnetic fields respectively.